Series ignition systems in contemporary internal combustion engines which are embodied as spark ignition engines have been operating for many decades according to the simple and reliable principle of coil discharge, i.e. an ignition coil which is configured as a transformer is charged partially as far as its saturation range on the primary side in accordance with its inductance from the on-board power system voltage. At the ignition time, the charge is interrupted by means of an electronic switching operation, for example by an ignition IGBT (Insulated Gate Bipolar Transistor). As a result, a voltage of, for example, 5 kV to 35 kV is built up on the secondary side and gives rise to a flashover in the spark gap of the spark plug in the combustion chamber of the internal combustion engine. The energy which is stored in the coil is subsequently dissipated in the ignition plasma.
In the course of the progressive development of engines, it has been necessary to implement reductions in terms of consumption and emissions, and in the last few years these have consequently placed an increasing additional burden on the ignition system and will continue to do so in the future. Examples of this are, for example, stratified combustion in which liquid fuel components with high flow rates impede the spark discharge and bring about numerous new spark formations. Rising combustion chamber pressures for improving the engine efficiency also increase the breakdown resistance in the spark gap and bring about an increase in the breakdown voltage which also influences the spark plug wear. In future highly charged engine generations the latter will give rise to secondary-side voltage increases far beyond 35 kV. Both the rising breakdown voltages and the flow states which become more intensive at the spark plug have a tendency to shorten the duration of the spark since ever larger proportions of the energy stored in the coil have to be made available to build up and maintain the spark. A most promising trend in the development of new combustion methods is the use of multiple sparks, wherein the coil energy is transmitted efficiently to the mixture at short intervals, which increases the inflammation reliability.
In application DE 10 2009 057 925.7, which was not published before the priority date of the present document, an innovative method for operating an ignition device for an internal combustion engine and an innovative ignition device for an internal combustion engine for carrying out the method are described. Accordingly, an ignition device for an internal combustion engine is formed with an ignition coil which is embodied as a transformer, a spark plug which is connected to the secondary winding of the ignition coil, a controllable switching element which is connected in series with the primary winding of the ignition coil, and a control unit which is connected to the primary winding of the ignition coil and to the control input of the switching element. The control unit makes available an adjustable supply voltage for the ignition coil and a control signal for the switching element as a function of the currents through the primary winding and the secondary winding of the ignition coil and as a function of the voltage between the connecting point of the primary winding of the ignition coil to the switching element and the negative terminal of the supply voltage. The method for operating this device has the following sequence in this context:
In a first phase (charging), the switching element is switched on by the control signal at a first switch-on time and switched off again at the predefined ignition time, in a subsequent second phase (breakdown), the primary voltage or a voltage derived therefrom is compared with a first threshold value, and when this voltage undershoots the first threshold value the switching element is switched on again at a second switch-on time,
in a subsequent third phase (arc) the supply voltage is regulated in such a way that the current through the secondary winding of the ignition coil corresponds approximately to a predefined current, and the current through the primary winding of the ignition coil is compared with a predefined second threshold value, and when this current overshoots the second threshold value the switching element is switched off again at a first switch-off time,
in a subsequent fourth phase (breakdown), the current through the secondary winding of the ignition coil is compared with a third threshold value, and when this current undershoots the third threshold value the switching element is switched on again at a third switch-on time,
the third and the fourth phase are, if appropriate, subsequently repeated until a predefined spark duration is reached at a time at which the switching element is definitively switched off.
A corresponding device is illustrated in FIG. 1, and the time profile of the significant voltages and currents is illustrated in FIG. 2.
However, a problem even with ignition devices of this type is the high breakdown voltage which is required for the first ignition in highly charged engines. It is known in this context that the breakdown voltage is lower in the case of negative polarity, that is to say if the positive potential of the supply voltage is applied to the ignition hook of the spark plug. On the other hand, under certain circumstances the inflammation of the mixture in the combustion chamber can be improved given a positive polarity.